Depends on the dams in question. There are two general approaches to building dams, which could be likened to the Great Pyramid or the World Trade Center. The former approach requires considerably more material but less critical engineering than the latter; the amount of additional material required increases greatly with height. The latter approach requires much less material, but the engineering is more critical. The material savings make the latter approach the only practical one past a certain height, but it has the disadvantage that damage to a small part of the structure(*) can destroy the whole thing.
To say a bomb could destroy some of the earthen dams would be akin to saying a bomb could destroy the Great Pyramids. Yes, bombs can destroy just about anything if they're big enough, but the size of bomb required would almost certainly be beyond what terrorists could acquire.
(*) This is not to imply the WTC centers were unreasonably weak. They withstood, for many minutes, damage and stresses far beyond anything they could have been expected to face; that they lasted as long as they did allowed many people to escape. My point was that the buildings are collapsed by damage to a relatively small percentage of their overall structure. The Great Pyramids, by contrast, would tend to be comparatively less succeptible to a global collapse caused by locallized damage (if nothing else because there isn't really anywhere for any collapsing material to fall).